Hitler was raised a Catholic. Stalin was raised in the Orthodox church. Pol Pot was a paranoid mysticist and Mao was Confucian. Atheism didn't make these people commit their crimes. How can it anyway? Atheism is not a moral philosophy. It demands nothing of anyone. It doesn't ask you to murder a hundred thousand Amalekites.
Do you know what the definition of religion is though? A relgion is, in the Oxford English, a system of continued ritual or reverence toward a supreme leader, deity or being, such systems usually demanding obedience, lip service, adoration and/or worship, especially as a means of achieving spiritual or material improvement.
What was Stalinism if not a forced system of reverence for a supreme leader where citizens were forced to pay lip service, to obey, and to show adorarion for Stalin, as a means of achieving material improvement (as contrast to capitalism)?
What was Nazi Germany if not the same, except the suprem leader in the equation was Hitler? Not to mention he fact Hitler had a highly delusional, genocidal superiority complex where Germans were concerned: he believed that those Deutschlandtrend descended from the Norse were Übermenschen. Superhumans.
Atheism contributed nothing to these genocides: Stalin's genocide was abot maintaining control, Hitler's genocide was about his belief that the Jews were subhuman and that Germany should rule the world. But Christianity? And Islam? These religions directly contributed to the Crusades (mandated by God, according to the Catholic church), to the Inquisitions (which were about torturing and murdering heretics) to the genocide of Native Americans (God's new chosen land), to the genocides of many African and Middle Eastern dictators. (Allah's will). I can think of no time in history where someone killed someone else for atheism. This is even reflected in prison populations. Most prisoners are religious. Atheists are highly unlikely to commit crimes.
It's a stone cold fact that the enforcement of some ideal is the single biggest cause of genocide in history. It's all about people forcing their beliefs or their crazy delusional ideas on other people. It's what Hitler did, it's what Stalin did, it's what the Popes have done, it's what the British and Columbus the Murdering Pirate did to the Native Americans. The list goes on. These people weren't murdered for atheism. Almost all of them were murdered for non-compliance with a despotic, tyrannical idea.